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Wed Apr 21, 2010 5:57 pm

the engine calculator gives me a 7.1 Static CR with my specs. (93x66) deck .017, CC+dish 70cc. Built with no base gasket/shim and .035 alum. head gasket.

This sounds a little low. I live about 1500 feet above sea level on the hillier east coast. Can I bump it up?

I was thinking thinner copper head gaskets, theyd have to be thin to make a difference of 5cc,,,,anyone know the thicknesses off hand?

Losing 5cc out of CC volume would get me 7.6 static CR. Thats about the best I can do without machining?

Wed Apr 21, 2010 8:42 pm

loose the head gaskets all together, they are a bad idea according to VW, and it will bump up your CR. all the new heads have no gasket, and come with a built in step to keep the CR stock.

simply lap the cyls to the head with valve lapping compound and it will seal.

Thu Apr 22, 2010 7:26 pm

Ok I can lap the top seal in build without gasket. ccs drop to 65, deck 017 yields 7.6:1.

I don't have experience with clearances. Is the .017 deck too little?

I could add a .010 base shim and increase deck to.027. yields 7.4 to one.

The suggestion is also on the table to have the heads flycut...to reduce cc's (inclusive of 10cc piston dish)

Will mock up the 1/2 side with cam and pushrods and see how close close is.

thanks for reading
Eric

Fri Apr 23, 2010 6:38 am

I am running a Raby 9550 cam and also larger valves with a 7.7 static C.R. and no issues, 87 Octane pump gas.

Fri Apr 23, 2010 7:49 am

.040-.035 is aboutthe tightest you can run.

Fri Apr 23, 2010 8:38 am

Ok shooting for .035deck height, adding .020 base shim=.037 deck 65ccs total CC vol, gives a 7.3:1. 9550 cam, L Jet FI, Pertonix

how does that CR sound? :?:
I just checked and machining costs are a little over 200.$ not unreasonable, and he will cc them.
Also I don't have a photo at the moment, but the 1/2 side barrels are touching fins where the barrels are close together. P/Cs were used successfully for 20k miles according to PO. No big deal to grind MORE cooling fin off. not my favorite thing, but its only .03" or < per barrel.

The 3/4 side has a good eighth inch clearance between barrels.
I guess poor casting QC, as these have only a small S cast on the last bottom fin I am not able to identify country of origin. :wink:

Also! after chasing the threads my 10M cylinder head bolts are way oversized after 35 years. New ones are a day away. Curious that the japanese tap was .009 bigger than the barrel studs major thread diameter measured.

Also! Does anybody have feedback on the Grant Company piston rings ?

thats it for now
thanks
Eric

Fri Apr 23, 2010 8:50 am

Do your heads have a step in them? This is important when figuring deck height. 7.3 is typical of what people get with dished pistons and stock head volumes.

Fri Apr 23, 2010 8:57 am

These heads don't have a step. Hey if 7.3:1 sounds good, two .020 shims it is!

Fri Apr 23, 2010 10:09 am

if you are running jakes cam kits, he knows exactly what CR you should run.

Fri Apr 23, 2010 1:00 pm

7.3 was factory on our 1977.

Mon Apr 26, 2010 9:16 am

yellow/exhaust cross section at intersection of valves and dish to piston flat blend point. .19"

red is intake. .15"

with dished piston it looks pretty good no? :)

can someone see any reason(s) to add base shims to accomplish the convention of .030/.050" deck height? I'm at .017" now which would give me a 7.9:1 compression ratio if I bolted together as it is.(this is a plan b really.)

plan a is to add .010 deck shims and result in 7.6:1
Plan c is to add .020 deck shims and have 7.3:1

thanks
Eric

Mon Apr 26, 2010 11:09 am

If you don't intend to rev the heck out of it you can probably get by with a .030 deck height. Others may chime in and say differently, I would rather have a tight but adequate deck than lower compression.

Mon Apr 26, 2010 11:23 am

IMHO 7.9 is a little high for a bus unless you want to burn premium. It will generate too much heat but the free increase in power will be nice.

Mon Apr 26, 2010 11:47 am

I had .020" machined off the top of my pistons to set my deck height. You effectively lose some cc in the overall CR equation giving a slightly higher CR vs just shimming . I can't remember how much it changed the volume of the dish and thus CR but even that small amount of machining did have an effect.

To estimate how much of a change, cc your piston dish and measure the depth of the dish to give you a cc per depth ratio. Just remember to mark the direction arrow on the underside of the piston before it gets machined off the top.

Mon Apr 26, 2010 2:05 pm

Oil Phil-M wrote: I had .020" machined off the top of my pistons to set my deck height. You effectively lose some cc in the overall CR equation giving a slightly higher CR vs just shimming . I can't remember how much it changed the volume of the dish and thus CR but even that small amount of machining did have an effect.

To estimate how much of a change, cc your piston dish and measure the depth of the dish to give you a cc per depth ratio. Just remember to mark the direction arrow on the underside of the piston before it gets machined off the top.

My limited experience with ACVW engines that had higher than stock compression is that they ran quite well and got good gas mileage. Never saw any overheating even on regular gas. I have both a set of flat head 93mm pistons and a set of domed 93 mm pistons that I want to try.

Mon Apr 26, 2010 2:57 pm

yeah this is becoming more exciting suddenly. a possible mistake-I'm going to mock it up but this time adjust the valves! it dawned on me there could be more lift since rockers were adjusted for the stock cam (smaller)D'OH.

Mon Apr 26, 2010 3:22 pm

Quote: My limited experience with ACVW engines that had higher than stock compression is that they ran quite well and got good gas mileage. Never saw any overheating even on regular gas. I have both a set of flat head 93mm pistons and a set of domed 93 mm pistons that I want to try.

the power and better mileage is free. It comes from the heat the compression generates. That gives you a little more expansion when the mixture burns. The other side of the equation is it generates more heat that has to be dealt with. On an ACVW heat is what we fear most.

You can get away with 8/1 but IMHO you will be on the edge if you get somewhere and the gasoline is cheap.

Mon Apr 26, 2010 3:47 pm

ok the idea of more torque is nice. no Wildthings we don't want to rev too much, but this bus has an automatic trans. so top end on the highway probably sees around 4000rpm.

well, the first temporary work around is easy. carrying octane booster then sounds like a good idea.

my current exhaust is the 75 smog afterburner with a pacesetter. not a horrible starting point. not the biggest heat bottleneck I could introduce a higher CR into.

IF temps still are high?

I have a 72/74 exhaust setup started. I haven't decided on muffler, need to fund the tin changeover too.If that system can drag some extra heat out?

if temps still are high~

if all just didn't work after a time~adding shims isn't impossible either.

of course this all supposes my heads haven't imploded already. :lol:

just trying to get through this year! aren't we all?

Eric

Tue Apr 27, 2010 12:23 pm

Quote: can someone see any reason(s) to add base shims to accomplish the convention of .030/.050" deck height? I'm at .017" now

If I'm understanding your thoughts correctly: Valve to piston clearence is good so why do I need to increase the deck?

Deck is the clearence between the piston and the cylinder head. Carbon buildup will decrease the clearence. Not sure if the thermal expansion comes out gaining or loosing clearence. To a lesser extent, over time as bearing and bushings wear the piston will top put ever so slightly higher. Too small of a deck clearence and the carbon encrusted piston will start to hit the carbon encrusted cylinder head at the top of each exhaust stroke.

Tue Apr 27, 2010 2:33 pm

A couple of things to bear in mind. Compression alone does not always create excess heat.

higher compression can do several things that will work to your advantage.....but you must tune for those advantages or you can end up with excessive heat.
(1) Running higher compression will allow you to run slightly less advance. That alone free's up a bit of power.
(2) When you run a higher compression, run the tightest deck you can safely work with. Large deck keeps most of the flame front down in the cylinder where (a) it heats the cylinder more and (b) burns slower so it transfers more heat to the head as well. Large deck always translates to higher heat...low or high compression.

The compression ratios were originally set on these vehicles primarily based around several things: the poor fuel of the day, the large bread loaf with low gearings and high revs that the engine is in and the just adequate ignition and restrictive exhaust that emmisions of the day dictated.

Keep a tight deck...go to a max of maybe 8.3:1, get a better exhaust, get a better coil, wires and plugs and check your distributors advance curve and modify it if necessary. Bump up to slightly larger valves for better breathing, get a better cam that breathes better than factory ....and if possible look at having a freeway flyer 4th gear installed that will take the highway rpms down a notch.

Many of these suggestions have already been made in this thread.
Be diligent in assembly. Make sure that the oil pressure relief valve seat is clean and seals and make sure you do not have excesss oil pressure keeping it open and bypassing the cooler is you use a larger pump. Spend extra money on balancing the entire rotating assembly. get solid rocker spacers and swivel feet adjusters.

All of this little etrxa effort adds up to an engine that iswell balanced and has lower friction. Added...it will make a large difference. Ray